Systems and methods for positioning multiple electrode structures in electrical contact with the myocardium

ABSTRACT

Systems and methods evaluate electrical contact between the myocardium and one or more electrodes inside the heart. The systems and methods electrically sense electrical contact between the myocardium and electrodes and generate unitary contact-indicating outputs indicating the presence or absence of electrical contact between the myocardium and each particular electrode. The systems and methods also correlate the electrode-specific unitary outputs to generate a compound contact-indicating output. The compound output represents the aggregate of the electrical contact between the myocardium and multiple electrodes on a multiple electrode array.

FIELD OF THE INVENTION

The invention relates to systems and methods for mapping and ablatingthe interior regions of the heart for treatment of cardiac conditions.

BACKGROUND OF THE INVENTION

Physicians make use of catheters today in medical procedures to gainaccess into interior regions of the body to ablate targeted tissueareas. It is important for the physician to be able to precisely locatethe catheter and control its emission of energy within the body duringtissue ablation procedures.

The need for precise control over the catheter is especially criticalduring procedures that ablate myocardial tissue from within the heart.These procedures, called electrophysiological therapy, are used to treatcardiac rhythm disturbances.

During these procedures, a physician steers a catheter through a mainvein or artery into the interior region of the heart that is to betreated. The physician then further manipulates a steering mechanism toplace the electrode carried on the distal tip of the catheter intodirect contact with the endocardial tissue. The physician directs energyfrom the electrode through myocardial tissue either to an indifferentelectrode (in a uni-polar electrode arrangement) or to an adjacentelectrode (in a bi-polar electrode arrangement) to ablate the tissue andform a lesion.

Physicians examine the propagation of electrical impulses in hearttissue to locate aberrant conductive pathways and to identify foci,which are ablated. The techniques used to analyze these pathways andlocate foci are commonly called "mapping."

Conventional cardiac tissue mapping techniques use multiple electrodespositioned in contact with epicardial heart tissue to obtain multipleelectrograms. These conventional mapping techniques require invasiveopen heart surgical techniques to position the electrodes on theepicardial surface of the heart.

An alternative technique of introducing multiple electrode arrays intothe heart through vein or arterial accesses to map myocardial tissue isknown. Compared to conventional, open heart mapping techniques,endocardial mapping techniques, being comparatively non-invasive, holdgreat promise. Multiple electrogram signals obtained from within theheart can be externally processed to detect local electrical events andidentify likely foci.

To achieve consistent, reliable foci identification rates, allelectrodes in a multiple electrode array should be in intimate,electrical contact with heart tissue. With invasive, open hearttechniques, multiple electrode contact on exposed epicardial surfacescan be visually confirmed directly by the physician. However, withcomparatively non-invasive endocardial mapping techniques, confirmingmultiple electrode contact within the beating heart can be problematic.

There is the need to provide simple, yet reliable ways of assuring thatthe electrodes of an endocardial multiple electrode structure are inintimate, electrical contact with tissue within the heart.

SUMMARY OF THE INVENTION

This invention has as its principal objective the realization of safeand efficacious endocardial mapping techniques.

One aspect of the invention provides a system and related method forevaluating electrical contact between the myocardium and at least twospaced apart electrodes in the heart. The system and method generate afirst unitary contact-indicating output indicating the presence orabsence of electrical contact between the myocardium and one of theelectrodes. The system and method also generate a second unitarycontact-indicating output indicating the presence or absence ofelectrical contact between the myocardium and the other electrode.

In a preferred embodiment, the system and method correlate theelectrode-specific unitary contact-indicating outputs to generate acompound contact-indicating output. The compound output indicates theaggregate of the electrical contact between the myocardium and multipleelectrodes in a multiple electrode array.

In one implementation, the system and method sense electrical contact byemitting through the electrodes an electrical signal that does notactivate the myocardium. In a preferred arrangement, the system andmethod acquire a tissue impedance measurement based upon this emission.

In a preferred implementation, the system and method sense electricalcontact by emitting through the electrodes an electrical signal thatactivates the myocardium, like a pacing pulse. In this implementation,the system and method acquire an electrogram based upon this signalemission.

In a preferred implementation, the system and method evaluate electricalcontact for a multiple electrode array that comprises electrodessupported on circumferentially spaced splines. The system and methodelectrically evaluate electrical contact to obtain a unitarycontact-indicating output for at least one electrode on each spline. Thesystem and method also correlate these unitary contact-indications toobtain a compound contact-indication, indicating the aggregateelectrical contact between the myocardium and the multiple electrodearray.

Another aspect of the invention electrically evaluates electricalcontact between the myocardium and an electrode inside the heart byemitting electrical energy into the myocardium through the electrode.The system and method that embody this aspect of the invention detect atleast one selected signal resulting from the emission of electricalenergy by the electrode. The selected signal varies with electricalcontact between the electrode and the myocardium to differentiatebetween electrical contact and the absence of electrical contact. Thesystem and method process the acquired signal by comparing it to anexpected signal. The system and method generate a contact-indicatingoutput for the electrode based upon the comparison. The contact-confirmoutput indicates the presence of electrical contact between theelectrode and the myocardium in the form of a contact-confirm signal,and a contactcontrary output indicates the absence of such contact.

Other features and advantages of the inventions are set forth in thefollowing Description and Drawings, as well as in the appended Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are plan views of an endocardial mapping probe having acollapsible multiple electrode structure whose electrical contact withthe myocardium can be evaluated using a contact sensing module thatembodies the features of the invention;

FIG. 3 is a somewhat diagrammatic view of the mapping probe shown inFIGS. 1 and 2 deployed in the left ventricle of the heart andelectrically coupled to a process controller that includes a contactsensing module that embodies the features of the invention;

FIG. 4 is a schematic view of a preferred implementation of a contactsensing module that embodies the features of the invention by sensingelectrical contact with the myocardium using pacing signals;

FIGS. 5 and 6 are schematic views of a switching device used inassociation with the contact sensing module shown in FIG. 4;

FIG. 7 is a flow chart showing the operation of the contact sensingmodule shown in FIG. 4 in Mode 1;

FIG. 8 is a flow chart showing the operation of the contact sensingmodule shown in FIG. 4 in Mode 2;

FIG. 9 is a schematic view of an alternative implementation of a contactsensing module that embodies the features of the invention by sensingelectrical contact with the myocardium using tissue impedancemeasurements;

FIG. 10 is an enlarged perspective view of a part of the multipleelectrode assembly usable in association with the contact sensing moduleshown in FIG. 9;

FIG. 11 is a flow chart showing the operation of the contact sensingmodule shown in FIG. 9, being used to sense individual electrode contactby tissue impedance measurements;

FIG. 12 is a flow chart showing further operation of the contact sensingmodule shown in FIG. 9, being used to assess composite contact of themultiple electrode structure by tissue impedance measurements;

FIG. 13 shows in somewhat diagrammatic form an alternative embodiment ofa collapsible endocardial multiple electrode structure deployed in theleft ventricle, being shown in a partially collapsed condition forurging electrical contact mostly with the blood pool;

FIG. 14 shows in somewhat diagrammatic form the alternative embodimentshown in FIG. 13, with the multiple electrode structure in an expandedcondition for urging electrical contact mostly with the myocardium;

FIG. 15 is a flow chart showing the operation of a contact sensingmodule usable in association with the multiple electrode structure shownin FIGS. 13 and 14 with the structure in its partially collapsedcondition for sensing blood path impedance;

FIG. 16 is a flow chart showing the operation of a contact sensingmodule usable in association with the multiple electrode structure shownin FIGS. 13 and 14 with the structure in its expanded condition forsensing tissue path impedance; and

FIG. 17 is a flow chart showing the operation of a contact sensingmodule usable in association with the multiple electrode structure shownin FIGS. 13 and 14 comparing sensed tissue path impedance and sensedblood path impedance to derive contact-indicating signals.

The invention may be embodied in several forms without departing fromits spirit or essential characteristics. The scope of the invention isdefined in the appended claims, rather than in the specific descriptionpreceding them. All embodiments that fall within the meaning and rangeof equivalency of the claims are therefore intended to be embraced bythe claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an endocardial mapping probe 10. The probe 10 comprises aflexible catheter tube 12 with a proximal end 14 and a distal end 16.The proximal end 14 carries an attached handle 18. The distal end 16carries a multiple electrode support assembly 20.

The multiple electrode support assembly 20 comprises an array offlexible spline elements 22 assembled to form a three dimensionalstructure. The far ends of the spline elements 22 radiate from a distalhub 24. The near ends of the spline elements 22 are affixed to a base26, which the distal end 16 of the catheter tube 12 carries.

Preferably, the spline elements 22 comprise thin, rectilinear strips ofresilient metal or plastic material. Still, other cross sectionalconfigurations can be used.

The support assembly 20 retains the spline elements 22 in acircumferentially spaced array. In the illustrated embodiment, the arraytakes the shape of a three dimensional basket structure. Of course, theresulting structure can assume other shapes.

The spline elements 22 carry an array of electrodes 38. Signal wires(not shown) electrically coupled to the electrodes pass along the guidetube 12 and connect to connectors 17 carried outside the handle 18.

In the illustrated and preferred embodiment, the probe 10 includes anouter sheath 40 carried about the catheter tube 12. As FIG. 2 bestshows, the sheath 40 has an inner diameter that is greater than theouter diameter of the catheter tube 12. As a result, the sheath 40slides along the catheter tube 12 (as the arrows in FIG. 2 show).

As FIG. 2 shows, forward movement advances the slidable sheath 40 overthe support assembly 20. In this position, the slidable sheath 40compresses and collapses the support assembly 20 for introductionthrough a vein or artery to the intended treatment site within the body.

As FIG. 1 shows, rearward movement retracts the slidable sheath 40 awayfrom the support assembly 20. This removes the compression force. Thefreed support assembly 20 opens and assumes its three dimensional shapeinside a heart chamber.

The electrode support assembly 20 can be assembled in different ways.Representative support assemblies are shown in co-pending U.S. patentapplication Ser. No. 08/206,414, filed Mar. 4, 1994, entitled "MultipleElectrode Support Structures," which is incorporated herein byreference.

FIG. 3 shows the support assembly 20 deployed and ready for use inside achamber 30 in the human heart. FIG. 3 generally shows the supportassembly 20 deployed in the left ventricle of the heart. Of course, theassembly 20 can be deployed in other regions of the heart, too. Itshould also be noted that the heart shown in the Figures is notanatomically accurate. The Figures show the heart in diagrammatic formto demonstrate the features of the invention.

As FIG. 3 shows, when deployed within the heart, the circumferentiallyspaced splines 22 make contact with circumferentially spaced regions ofthe endocardium.

As FIG. 3 also shows, the connectors 17 plug into a processing system32. The system 32 includes an on board signal processing module 34 thatreceives and processes diagnostic signals from the multiple electrodes38 on the assembly 20. The output of the module 34 helps the physicianin identifying appropriate ablation sites within the heart. The type ofsignals that the module 34 receives and processes can vary, according tothe choice of the physician.

For example, the physician can condition the module 34 to take multiple,sequential measurements of the transmission of electrical current byheart tissue to obtain tissue resistivity measurements. The processingof tissue resistivity signals to identify appropriate ablation sites isdisclosed in co-pending U.S. patent application Ser. No. 08/197,236,filed Jan. 28, 1994, and entitled "Systems and Methods for MatchingElectrical Characteristics and Propagation Velocities in Cardiac Tissueto Locate Potential Ablation Sites."

Alternatively, or in conjunction with tissue resistivity measurements,the physician can condition the module 34 to acquire and processelectrograms in a conventional fashion. The module 34 processes theelectrogram information to map the conduction of electrical impulses inthe myocardium.

Furthermore, if desired, the physician can condition one or more of theelectrodes 38 to emit a therapeutic signal into the myocardium. Forexample, the physician can condition selected electrodes 38 to emitradio-frequency energy to ablate myocardial tissue. Alternatively, aseparate ablation probe (not shown) can be deployed for this purpose.

Regardless of the particular type of signal that is processed, thesupport assembly 20 should be oriented in the heart chamber 30 to holdall or substantially all electrodes 38 in electrical contact themyocardium. According to the invention, the controller 32 includes atissue contact sensing module 36. The contact sensing module 36evaluates electrical contact between the myocardium and the multipleelectrode array inside the heart.

As will be described in greater detail later, the contact sensing module36 electrically senses electrical contact between the myocardium and atleast one electrode 38 on spaced apart spline elements 22. The module 36generates a unitary contact-indicating output for the energy emittingelectrodes 38. The unitary output indicates the presence or absence ofthe electrical contact between a particular electrode and themyocardium.

In the illustrated and preferred embodiment, the module 36 alsocorrelates the electrode-specific unitary contact-indicating outputs togenerate a compound contact-indicating output. The compound outputindicates the aggregate electrical contact between the myocardium andthe multiple electrode array.

In the preferred embodiment the module 36 electrically senses electricalcontact by emitting from one or more of the electrodes 38 an electricalsignal that activates the myocardium. In this embodiment, the module 36ascertains electrical contact by detecting electrograms.

In another embodiment, the module 36 electrically senses electricalcontact by emitting from one or more of the electrodes 38 an electricalsignal that does not activate the myocardium. In this arrangement, themodule 36 ascertains electrical contact by measuring tissue impedance.

I. Pace-Locate Module

FIGS. 4 to 6 show a preferred implementation of the contact sensingmodule (designated 36(1)) and the associated processing system 32.

In this implementation, the module 36(1) generates pacing signals toevaluate the quality of electrical contact between the electrodes 38 andthe myocardium. This sensing information is preferably obtained bypacing at the end of diastole, at the stage of the cardiac cycle whenthe heart is at its maximum relaxation.

In this implementation (see FIG. 4), the processing system 32 includes ahost central processing unit (CPU) 42 (as FIG. 3 also shows). The CPU 42communicates with a mass storage device 44 and an extended static RAMblock 46. A user interactive interface 48 also communicates with the CPU

As FIG. 4 shows, the interactive user interface 48 includes an inputdevice 50 (for example, a key board or mouse) and an output displaydevice 52 (for example, a graphics display monitor or CRT).

The CPU 42 communicates with both the signal processing module 34 andthe contact sensing module 36(1). The CPU 42 coordinates the controlfunctions for the modules 34 and 36(1).

In the illustrated and preferred implementation (as FIG. 4 shows), thecontact sensing module 36(1) includes a controller interface 54 coupledto the host processor 42. The controller interface 54 is also coupled toa pulse generator 56 and an output stage 58.

The output stage 58 is electrically coupled by supply path 60 and returnpath 62 to a switching element 64. The configuration of the switchingelement 64 can vary. FIG. 5 diagrammatically shows one preferredarrangement.

FIG. 5 shows for illustration purposes a spline 22 with seven adjacentelectrodes 38, designated E1 to E7. Each electrode E1 to E7 iselectrically coupled to its own signal wire, designated W1 to W7. Anindifferent electrode, designated EI in FIG. 5, is also electricallycoupled to its own signal wire WI. The indifferent electrode EI ispreferably an electrode in the blood pool. However, the indifferentelectrode EI can be formed by other means. For example, it can also be aconventional patch electrode attached on the outside of the patient'sbody.

In this arrangement, the switching element 64 includes an electronicswitch S_(M) and electronic switches S_(E1) to S_(E7) that electricallycouple the pacing output stage 58 to the signal wires W1 to W7. Theswitch S_(M) governs the overall operating mode of the electrodes E1 toE7 (i.e., unipolar or bipolar). The switches S_(E1) to S_(E7) govern theelectrical conduction pattern of the electrodes E1 to E7.

The switches S_(M) and S_(E1) to E7 are electrically coupled to theoutput stage 58 of the pulse generator 56 through the lines 60 and 62.The supply line 60 of the output stage 58 is electrically coupled to theleads L1 of the switches S_(E1) to E7. The return line 62 of the outputstage 58 is electrically coupled to the center lead L2 of the modeselection switch S_(M). A connector 66 electrically couples the leads L3of the switches S_(M) and S_(E1) to E7.

The center leads L2 of the selecting switches S_(E1) to E7 are directlyelectrically coupled to the signal wires W1 to W7 serving the electrodesE1 to E7, so that one switch S_(E)(N) serves only one electrodeE.sub.(N).

The lead L1 of the switch S_(M) is directly electrically coupled to thesignal wire WI serving the indifferent electrode EI.

An interface 68 electronically sets the switches S_(M) and S_(E1) to E7among three positions, designated A, B, and C in FIG. 6.

As FIG. 6 shows, Position A electrically couples leads L1 and L2 of theassociated switch. Position C electrically couples leads L2 and L3 ofthe associated switch. Position B electrically isolates both leads L1and L3 from lead L2 of the associated switch.

Position B is an electrically OFF position. Positions A and B areelectrically ON positions.

By setting switch S_(M) in Position B, the interface 68 electronicallyinactivates the switching network 64.

By setting switch S_(M) in Position A, the interface 68 electronicallyconfigures the switching element for operation in the unipolar mode. Thecenter lead L2 of switch S_(M) is coupled to lead L1, electronicallycoupling the indifferent electrode EI to the return 62 of the pulseoutput stage 58. This configures the indifferent electrode EI as areturn path for current.

With switch S_(M) set in Position A, the interface 68 electronicallyselectively configures each individual electrode E1 to E7 to emitcurrent by sequentially setting the associated switch S_(E1) to E7 inPosition A. When the selected electrode E1 to E7 is so configured, it iselectronically coupled to the supply 60 of the pulse output stage 58 andemits current. The indifferent electrode EI receives the currentsequentially emitted by the selected electrode E1 to E7.

By setting switch S_(M) in Position C, the interface 68 electronicallyisolates the indifferent electrode EI from the electrodes E1 to E7. Thisconfigures the switching element for operation in the bipolar mode.

With switch S_(M) set in Position C, the interface 68 can electronicallyalter the polarity of adjacent electrodes E1 to E7, choosing amongcurrent source, current sink, or neither.

By setting the selected switch S_(E1) to E7 in Position A, the interface68 electronically configures the associated electrode E1 to E7 to be acurrent source. By setting the selected switch S_(E1) to E7 in PositionC, the interface 68 electronically configures the associated electrodeE1 to E7 to be a current sink. By setting the selected switch S_(E1) toE7 in Position B, the interface 68 electronically turns off theassociated electrode E1 to E7.

The controller interface 54 of the module 36(1) includes control buses70, 72, and 74. Bus 70 conveys pulse period control signals to the pulsegenerator 56. Bus 72 conveys pulse amplitude control signals to thepulse generator 56. Bus 74 constitutes the control bus path for theswitching element 64 by the means of interface 68.

When used to pace the heart, the switching element 64 distributes pacingsignals generated by the pulse generator 56 to selected basketelectrodes 38. The pacing sequence is governed through the interface 54by the host processor 42.

When emitted by a selected electrode 38 in electrical contact withviable myocardium, the pacing signal depolarizes myocardial tissue atthe site of the selected electrode 38. Since the intensity of theelectric field generated by the pacing signal decreases with the squareof the distance from the emitting electrode 38, the pacing signal willnot be effective unless the emitting electrode 38 is very near or inintimate contact with viable myocardium.

The basket electrodes 38 will sense signals as the depolarization frontgenerated by the pacing signal reaches them. These signals are passedback through the switching element 64 and the data acquisition system 75to the host processor 42 for analysis according to prescribed criteria,which will be described in greater detail later. The analysis generatescontact-indicating output. The contact-indicating output reflects thequality of electrical contact that exists between the myocardium and oneor more electrodes 38 on the support assembly 20.

In the illustrated and preferred implementation, the module 36(1)operates in two modes. In the first mode, the module 36(1) operates toestablish a reliable electrode pacing site by assessing the quality ofelectrical contact of a single, selected electrode 38 on the supportassembly 20 (which will also be called a "unitary contact-indicatingoutput."). In the second mode, the module 36(1) generates pacing signalsin succession from multiple electrode pacing sites to generate thecontact-indicating output, which assesses the quality of electricalcontact for a composite of all the electrodes on the support assembly 20(which will also be called a "compound contact-indicating output").

(i) Mode 1: Establishing Unitary Contact

In Mode 1 (see FIG. 7), the CPU 42 selects one electrode 38 on thesupport structure 20. Through the controller interface 54, the CPU 42causes the pulse generator 56 to generate a pacing signal through theswitching element 64 to the selected electrode.

In Mode 1, the pacing signal must provide enough voltage or current tothe selected electrode to locally stimulate the myocardium. Still, thepacing signal should not be large enough to field stimulate themyocardium at a distance greater than about 2 mm. In the preferredimplementation, it is believed that the pacing signal should be about 3milliamps (3 Volts), with a pulse width of about 0.5 msec.

By analyzing signals received back through the switching element 64 fromat least one electrode on the support assembly 20, the CPU 42 confirmscapture; that is, the CPU 42 confirms that a depolarization wavefrontemanated from the selected electrode in response to the pacing signal.In the preferred implementation, the CPU 42 requires all or at least asignificant number of electrodes spaced from the selected electrode tosense capture. The greater the number of sensing electrodes used tosense capture, the more reliable the overall output will be.

Upon sensing capture at the selected electrode, the CPU 42 generates aunitary contact-confirm signal. The unitary contact-confirm signalindicates that the selected electrode is in sufficient electricalcontact with viable myocardium. The CPU 42 then automatically switchesto the module 36(1) to Mode 2 operation.

In the preferred implementation of Mode 1, multiple pacing signals aresent to the selected electrode and multiple captures are confirmed. Thisconfirms that the module 36(1) is working reliably, and that theelectrical stimulus of the pacing signal is large enough to proceed toMode 2 operation.

If signals received back through the switching element 64 from at leastone electrode on the support assembly 20, do not confirm capture, theCPU 42 generates a unitary contact-contrary signal. The unitarycontact-contrary signal indicates either (i) the selected electrode isnot in sufficient electrical contact with the myocardium, or (ii) thatthe myocardium that the selected electrode contacts is not viable. Theunitary contact-contrary signal advises the physician to relocate thesupport structure 20 and repeat Mode 1 until a unitary contact-confirmsignal is received or he/she decides that the myocardium is not viable.

(ii) Mode 2: Establishing Compound Contact

Upon entering Mode 2 (see FIG. 8), the CPU 42 causes the controllerinterface 54 to perform a prescribed pacing cycle. During the pacingcycle, the switching element 64 sends a pacing signal in succession to aselected number of electrodes 38. In the preferred implementation ofMode 2, every electrode on the support structure receives at least onepacing signal during the pacing cycle.

The pacing rate must be faster than the baseline heart beat (that is,typically greater than about 70 beats per minute). Preferably, thepacing rate should be at least 20% larger than the baseline heart beat(that is, typically greater than 84 beats per minute).

For example, a pacing rate of 120 pacing signals per minute could beselected. Given 60 pacing signals during each pacing cycle (for asupport assembly 20 having 60 electrodes), this selected pacing ratewould require 30 seconds to complete one pacing cycle.

The CPU 42 registers the number of captures sensed during the pacingcycle. The CPU 42 then evaluates, for each pacing cycle, the number ofregistered captures against predetermined criteria. Based upon thisevaluation, the signal processing system generates the compoundcontact-indicating output.

In the preferred implementation, the CPU 42 compares, for each pacingcycle, the number of registered captures to the maximum possible numberof captures. From this comparison, the signal processing system createsa contact value C_(VAL), which is computed as follows: ##EQU1## where

C_(REG) is the actual number of registered captures during the pacingcycle, and

C_(MAX) is the maximum number of captures possible during the pacingcycle, which corresponds to the number pacing signals generated duringthe pacing cycle.

In this implementation, the CPU 42 compares the contact value C_(VAL) toa predetermined value C_(EST) to generate the compoundcontact-indicating output.

If the contact value C_(VAL) equals or exceeds the predetermined valueC_(EST), the CPU 42 generates through the output 52 a compoundcontact-confirming signal. The compound contact-confirming signalindicates to the physician that the electrodes 38 on the supportstructure 20 are in sufficient contact with the myocardium to perform areliable mapping procedure.

If the contact value C_(VAL) lies below the predetermined value C_(EST),the CPU 42 generates through the output 52 a compound contact-contrarysignal. The compound contact-contrary signal indicates to the physicianthat the electrodes 38 on the support structure 20 are not in sufficientelectrical contact with the myocardium to perform a reliable mappingprocedure. The combined contact-contrary signal suggests that thephysician should relocate the support assembly and repeat Mode 1.

In either situation, in Mode 2 operation, the module 36(1) allows thephysician to generate another pacing cycle to confirm the compoundcontact-indicating output.

The predetermined capture value C_(EST) can vary according to thephysiology of the patient, the structure of the electrode supportassembly, and the medical judgement of the physician. For example,C_(EST) for a patient having entirely viable myocardium can be largerthan for a patient having damaged heart tissue. The C_(EST) for anassembly 20 having a larger number of electrodes 38 can be smaller thanfor an assembly 20 having a smaller number of electrodes 38. The medicaljudgement of one physician may require a larger C_(EST) than anotherphysician.

Taking these variables into consideration, it is believed that anacceptable C_(EST) lies in the range of about 50 to 100. A realistictarget for C_(EST) is believed to lie at or above 85 for mostprocedures.

In the preferred implementation, pacing is accomplished by operating theelectrodes 38 in a uni-polar configuration (that is, by setting switchS_(M) in Position A in FIG. 5). In this configuration, an externalindifferent electrode EI serves as the return path for the pacingsignal.

In an alternative implementation, the pacing could be accomplished byoperating the electrodes 38 in a bi-polar configuration (that, bysetting switch S_(M) in Position C in FIG. 5). In this configuration,the pacing signal is generated between spaced apart pairs of electrodes38 on the support assembly 20 using bipolar pacing techniques.Preferably, one of the electrodes 38 of the bipolar pair is known not tobe in electrical contact with viable myocardium. Pacing artifacts arereduced using bipolar pacing.

Once the module generates a compound contact-confirming signal duringMode 2, the physician can cause the process controller 32 to switch tothe signal processing module 34 and proceed with the intended mappingprocess.

Once a satisfactory compound contact-confirming signal is received, theCPU 42 can also specifically identify the electrode sites where captureis sensed and not sensed. By operating the module 36(1) in Mode 2through successive pacing cycles, the CPU 42 can generateelectrode-specific information. For example, the CPU 42 can processhistorical information obtained during successive pacing cycles toidentify locations where capture is consistently not sensed. Bygenerating an output identifying those electrode locations where captureis consistently not sensed, the CPU 42 provides the physician withinformation that, on a gross scale, indicates where regions of nonviable(i.e., infarcted) myocardium exist.

II. Tissue Impedance-Locate Module

FIG. 9 shows an alternative implementation of the contact sensing module(designated 36(2)). In this embodiment, the module 36(2) derivescontact-indicating information by measuring tissue impedance.

The module 36(2) offers the physician the capability of sensing contactthroughout the cardiac cycle, systole and diastole. The module 36(2)also simplifies sensing contact at each individual electrode site.

As FIG. 9 shows, the module 36(2) includes an oscillator 76 thatgenerates a sinusoidal voltage signal. An associated interface 78 has abus 80 that controls the frequency of the output voltage signal and abus 82 that controls the amplitude of the output voltage signal. Theinterface 78, in turn, is programmed by the host CPU 42, alreadydescribed.

The oscillator 76 has as an output stage that includes avoltage-to-current converter 84. In conventional fashion, the converter84 converts the sinusoidal voltage signal to current.

In the illustrated and preferred embodiment, the transmitted current hasan amplitude of about 0.1 milliamps to 5.0 milliamps. The lower range ofthe current amplitude is selected to be high enough to overcome theinfluence of the double layer at the tissue-electrode interface on theimpedance measurement. The high range of the current amplitude isselected to avoid the induction of fibrillation.

The current has a frequency in a range of about 5 to 50 kHz. The rangeis selected to avoid the induction of fibrillation, as well as providecontrast between infarcted tissue and healthy tissue. The output of theconverter 84 can comprise a constant current with a constant frequencywithin the above range. Alternatively, the interface 78 can control themodulation of the frequency of the current signal within the prescribedrange.

The current output of the module 36(2) is supplied to the basketelectrodes 38 via supply path 86 through the switching element 64,already described and shown in FIG. 5. The interface 78 electronicallyconfigures the switching element 64 to direct current in succession toselected basket electrodes 38 through their associated signal wires in aunipolar mode (that is, with switch S_(M) in Position A). When operatedin a unipolar mode, the current return path 88 to the switch element 64is provided by the indifferent electrode EI attached to the patient.Line 90 constitutes the control bus for the switching element 64.

The module 36(2) includes a data acquisition system 92. While current isemitted by each selected basket electrode 38, the system 92 senses thevoltage in the tissue path lying between the selected electrode 38 andthe indifferent electrode EI. Based upon the data acquired by the system92, the host processor 42 computes the impedance of the path (in ohms)lying between the electrodes.

The measured path impedance can be directly correlated to electrodecontact with tissue. A relatively high impedance value indicateselectrical contact with healthy myocardium, while a relatively lowimpedance value indicates either a lack of electrical contact or thepresence of infarcted myocardium in the path.

Preliminary tests indicate that for good contact the impedance withrespect to a 40 cm² patch electrode on the back of the patient is about200 ohms. For poor contact the impedance is about 125 ohms. Other valuescan be obtained for different electrode geometries.

The CPU 42 analyzes the computed impedance values according toprescribed criteria. Based upon this comparison, the CPU 42 generates acontact-indicating output. The contact-indicating output reflects theamount of electrical contact that exists between the myocardium and oneor more electrodes 28 on the support assembly 20.

In this implementation (as FIG. 10 shows), the electrodes 38 are mountedto each spline 22 to maximize surface contact with endocardial tissue,while at the same time minimizing exposure to the surrounding bloodpool. Incidental exposure of the electrodes 38 to blood while in contactwith heart tissue introduces an unwanted artifact to tissue impedancemeasurement, because the resistivity of blood is about three times lowerthan the resistivity of heart tissue. This artifact can skew theimpedance measurement to a lower value, thereby reducing the desiredcontrast between contact-confirm and contact-contrary signals.

In the illustrated embodiment, the electrodes 38 are made of platinum orgold plated stainless steel bands affixed to only one side of therectilinear splines 22. This is the side of the spline 22 that, in use,contacts endocardial tissue. The opposite surface of the splines 22(which, in use, contacts the blood pool) is free of electrodes.

It is believed that no more than 20% of the electrode surface should beexposed to the blood pool during use Preferable, less than 5% of theelectrode should be so exposed during use.

(i) Determine Absolute Tissue Contact

One preferred implementation of this embodiment (see FIG. 11) maintainscontact between the support assembly 20 and the myocardium whilederiving the impedance values.

The implementation computes tissue path impedance using the followingequation: ##EQU2##

The PathVoltage and PathCurrent are both root mean squared (RMS) values.

The voltage is measured in succession between each electrode and theindifferent electrode (or between EI and E(n), where n represents thelocation of the current emitting electrode). The impedance computedreflects not only the impedance of the underlying myocardium, but alsoincludes the impedance of the other tissue mass in the path. Thecomputed impedance therefore is not the actual impedance of themyocardium itself. Rather, it provides a relative scale of impedancedifferences of the myocardium lying in contact with the electrodes.

The CPU 42 compares the computed impedance E_(VAL) for each electrode toa prescribed impedance value E_(EST).

The value E_(EST) should account for the resistivities of blood andviable myocardium at the frequency of the emitted current. For example,blood resistivity is about 150 ohm·cm, while viable myocardiumresistivity is about 450 ohm·cm (infarcted myocardium resistivity isabout 250 ohm·cm). If the indifferent electrode EI is an external 40 cm²patch electrode, E_(EST) is about 150 ohms. Other values could be usedfor other electrode geometries. If the computed impedance value E_(VAL)equals or exceeds the prescribed value E_(EST), the CPU 42 inferssufficient electrical contact at that electrode site and generates aunitary contact-confirm signal. If the computed impedance value E_(VAL)is less than the prescribed value E_(EST), the processing system infersa lack of sufficient electrical contact at that electrode site andgenerates a unitary contact-contrary signal.

The unitary contact-confirm and contactcontrary signals are specific toeach electrode site. The physician can use each of these site specificsignals to position the support assembly to obtain the best sitespecific electrical contact.

Alternatively, or at the same time (see FIG. 12), the CPU 42 providesthe physician with an overall assessment of electrode contact for thesupport assembly. In this implementation, if the computed impedancevalue E_(VAL) equals or exceeds the prescribed value E_(EST), the CPU 42assigns a position score value of 1, indicating sufficient electricalcontact at that electrode site. If the computed impedance value E_(VAL)is less than the prescribed value E_(EST), the CPU 42 assigns a positionscore value of 0, indicating the lack of sufficient electrical contactat that electrode site.

The CPU 42 computes the sum of the position score values obtained (ΣPS).The CPU 42 then compares the position score value sum ΣPS to the totalnumber of electrodes sampled (ΣE). From this comparison, the CPU 42creates a contact value C_(VAL), which is computed as follows: ##EQU3##

In this implementation, as with the previously described embodiment, theCPU 42 compares the contact value C_(VAL) to a predetermined valueC_(EST) to generate the compound contact-indicating output.

If the contact value C_(VAL) equals or exceeds the predetermined valueC_(EST), the CPU 42 generates a compound contact-confirming signal. Thecompound contact-confirming signal indicates to the physician thatenough electrodes 38 on the support structure 20 are in sufficientelectrical contact with the myocardium to perform a reliable mappingprocedure.

If the contact value C_(VAL) lies below the predetermined value C_(EST),the CPU 42 generates a compound contact-contrary signal. The compoundcontact-contrary signal indicates to the physician that the electrodes38 on the support structure 20 are not in sufficient contact with themyocardium to perform a reliable mapping procedure. The compoundcontact-contrary signal-suggests that the physician should relocate thesupport assembly 20.

As before stated, the predetermined contact value C_(EST) can varyaccording to the physiology of the patient, the structure of theelectrode support assembly 20, and the medical judgement of thephysician.

For reasons stated above, it is believed that an acceptable C_(EST) liesin the range of about 50 to 100. A realistic target for C_(EST) isbelieved to lie at or above 85 for most procedures.

In either situation, the module 36(2) allows the physician to generateanother cycle to confirm the compound contact-indicating output.

(ii) Comparative Blood/Tissue Impedance

FIGS. 13 and 14 show an alternate embodiment of a multiple electrodeassembly 94 whose contact with the myocardium can be electricallydetermined by differential tissue impedance measurements.

Like the assembly 20 shown in FIG. 1, the assembly 94 comprises an arrayof flexible spline elements 96 assembled to form a three dimensionalstructure carried on the distal end 16 of the catheter tube 12. Like theassembly 20, the spline elements 96 of the assembly 94 carry an array ofelectrodes 98. The electrodes 98 are mounted to each spline 22 tomaximize surface contact with endocardial tissue, while at the same timeminimizing exposure to the surrounding blood pool, as FIG. 10 shows.

Unlike the assembly 20, which used the outer sheath 40 to collapse thespline elements 22, the assembly 94 includes a center wire 100 to alterthe shape of the structure. The distal end of the wire 100 connects tothe hub 102 of the assembly 94. The wire 100 passes through the cathetertube 12 into the handle 104, where its proximal end attaches to acontrol lever 106.

As FIG. 13 shows, pushing the control lever 106 forward causes the wire100 to straighten the splines elements 96 inward. This causes at least apartially collapse or elongation of the support assembly 94 within theheart chamber 30. This in turn pulls the electrodes 98 away fromelectrical contact with the myocardium. Instead, the electrodes 98 aremostly exposed to the blood pool in the chamber 30.

As FIG. 14 shows, pulling back on the control lever 106 causes the wireto bend the spline elements 96 outward. The support assembly 94 expandsto its intended fully deployed condition, urging the electrodes 94toward electrical contact with the myocardium.

Used in association with the support assembly 94, the contact sensingmodule 36(2) is conditioned by the CPU 42 to measure the relative changein impedance values when the support assembly 94 is opened to contactheart tissue (as in FIG. 14) and when the support assembly 94 ispartially collapsed to contact blood (as in FIG. 13). The CPU 42generates a contact-indicating output based upon the measured change inimpedance values.

As FIG. 15 shows, the CPU 42 prompts the physician to move the controllever 106 forward to urge the electrodes 98 into the blood pool (as FIG.13 shows). With the support assembly 98 in this condition, the CPU 42conditions the module 36(2) to transmit an electrical current insuccession through all or at least a significant number of electrodes98, in the manner previously described. The CPU 42 computes theimpedance, which represents blood path impedance BloodPathImp, sincemost of the electrode 98 are exposed to the blood pool. BloodPathImp iscalculated as follows: ##EQU4##

The BloodPathVoltage and BloodPathCurrent are both root mean squared(RMS) values.

The voltage is measured in succession between each electrode and theindifferent electrode (or between EI and E(n), where n represents thelocation of the current emitting electrode).

As FIG. 16 shows, the CPU 42 then prompts the physician to move thecontrol lever 106 rearward to urge the electrodes 98 into electricalcontact with the myocardium (as FIG. 14 shows). With the supportassembly 98 in this condition, the CPU 42 conditions the module 36(2) toagain transmit an electrical current in succession through the sameelectrodes that emitted current when the support assembly 94 wascollapsed in the blood pool. The CPU 42 computes impedance, which thistime represents tissue path impedance TissuePathImp.

The CPU 42 computes TissuePathImp using the following equation: ##EQU5##

As before, the TissuePathVoltage and TissuePathCurrent are both rootmean squared (RMS) values.

As before, the voltage is measured in succession between each electrodeand the indifferent electrode (or between EI and E(n), where nrepresents the location of the current emitting electrode).

As FIG. 17 shows, the CPU 42 compares the derived TissuePathImpedancefor each electrode with the derived BloodPathImpedance for thatelectrode. The processing system derives the change in the impedance(ΔIMP), as follows: ##EQU6##

For each electrode 98 that emitted current, the CPU 42 compares ΔIMP toa prescribed difference value ΔEST. If ΔIMP equals or exceeds ΔEST, theCPU 42 infers sufficient electrical contact between the electrode andthe myocardium. In this circumstance, the CPU 42 generates a unitarycontact-confirm signal. If ΔbIMP is less than ΔEST, the CPU 42 infersthat sufficient electrical contact does not exist. In this circumstance,the CPU 42 generates a unitary contact-contrary signal.

The unitary contact-confirm and contactcontrary signals in thisimplementation are also specific to each electrode. The physician canuse each of these site specific signals to position the support assemblyto obtain the best site specific electrical contact.

Alternatively, or at the same time, the CPU 42 can provide the physicianwith an overall assessment of electrode contact for the support assembly98. In this implementation (see FIG. 17), if ΔIMP equal or exceeds ΔEST,the CPU 42 assigns a position score value of 1, indicating sufficientelectrical contact at that electrode site. If ΔIMP is less than ΔEST,the CPU 42 assigns a position score value of 0, indicating the lack ofsufficient electrical contact at that electrode site.

The CPU 42 then computes the sum of the position score values obtained(ΣPS), as before described and as shown in FIG. 12. Also as beforedescribed and shown in FIG. 12, the CPU 42 then compares the positionscore value sum ΣPS to the total number of electrodes sampled (ΣE). Fromthis comparison, the signal processing system creates a contact valueC_(VAL), which is compared to the predetermined contact value C_(EST) togenerate either a position contrary signal or a position confirm signal,as previously described. As before stated, a realistic value for C_(EST)for most procedures is believed to lie at or above 85.

The features of the invention are set forth in the following claims.

We claim:
 1. A system for evaluating electrical contact between themyocardium and a multiple electrode array inside the heart, the multipleelectrode array comprising first and second circumferentially spacedsplines for contacting circumferentially spaced endocardial regions,each spline supporting at least one electrode, the systemcomprisingfirst means for electrically sensing electrical contactbetween at least one electrode on the first spline and the myocardiumand for generating a first unitary contact-indicating output indicatingthe presence or absence of the electrical contact, and second means,operable independently of the first means, for electrically sensingelectrical contact between at least one electrode on the second splineand the myocardium and for generating a second unitarycontact-indicating output indicating the presence or absence of theelectrical contact, at least one of the first and second means beingoperable to sense electrical contact by emitting from the respectiveelectrode an electrical signal that activates the myocardium.
 2. Asystem according to claim 1and further including third means forcorrelating the first and second unitary contact-indicating outputs togenerate a compound contact-indicating output indicating the aggregateelectrical contact between the myocardium and the multiple electrodearray.
 3. A system according to claim 1wherein the one first or secondmeans emits a pacing signal from the respective electrode and senseselectrical contact by sensing electrograms.
 4. A system according toclaim 1and further including means for conditioning at least oneelectrode on the first and second splines to emit a therapeutic signalinto the myocardium.
 5. A system according to claim 1and furtherincluding means for conditioning at least one electrode on the first andsecond splines to obtain a diagnostic signal from the myocardium.
 6. Asystem for evaluating electrical contact between the myocardium and amultiple electrode array within the heart, the multiple electrode arraycomprising first and second circumferentially spaced splines forcontacting circumferentially spaced endocardial regions, each splinesupporting at least one electrode, the system comprisingan energygenerating element for electrical connection to at least one electrodeon each support spline to emit electrical energy into the myocardiumthrough the electrodes, and a signal acquisition element adapted to beelectrically coupled to the emitting electrodes for detecting,independently for each emitting electrode, at least one acquired signalresulting from the emission of electrical energy by the respectiveelectrode, the acquired signal varying with electrical contact betweenthe respective emitting electrode and the myocardium to differentiatebetween the presence of the electrical contact and the absence of theelectrical contact.
 7. A system according to claim 6and furtherincluding means for comparing the acquired signal for each emittingelectrode with an expected signal to determine the presence or absenceof the electrical contact.
 8. A system according to claim 6and furtherincluding a processing element electrically coupled to the signalacquisition element that, independently for each emitting electrode,compares the acquired signal for each emitting electrode with anexpected signal.
 9. A system according to claim 8and further includingan output element for generating, independently for each emittingelectrode, a unitary contact-indicating output for the respectiveemitting electrode based upon the comparison, the unitarycontact-indicating output indicating the presence or absence of theelectrical contact.
 10. A system according to claim 9wherein the outputelement generates, independently for each emitting electrode, a unitarycontact-confirm output for the respective electrode if the acquiredsignal for the respective electrode compares with the expected signaland a unitary contact-contrary output for the respective electrode ifthe acquired signal does not compare with the expected signal.
 11. Asystem according to claim 9wherein the processing element correlates theindependent unitary contact-indicating output for each emittingelectrode to derive a compound contact-indicating output indicating theaggregate of the electrical contact for the multiple electrode array.12. A system according to claim 6wherein the energy generating elementgenerates electrical energy that does not activate the myocardium.
 13. Asystem according to claim 12wherein the generated electrical energy isan electric current, and wherein the signal acquisition element detectstissue impedance.
 14. A system according to claim 6wherein the energygenerating element generates electrical energy that activates themyocardium.
 15. A system according to claim 14wherein the generatedelectrical energy is a pacing signal, and wherein the signal acquisitionelement detects electrograms.
 16. A system according to claim 6andfurther including means for conditioning at least one electrode on thefirst and second support splines to emit a therapeutic signal into themyocardium.
 17. A system according to claim 6and further including meansfor conditioning at least one electrode on the first and second supportsplines to obtain a diagnostic signal from the myocardium.
 18. A systemfor evaluating electrical contact between the myocardium and anelectrode inside the heart comprisingan energy generating element forelectrical connection to the electrode to emit electrical energy intothe myocardium through the electrode that activates the myocardium, asignal acquisition element adapted to be electrically coupled to theelectrode for detecting at least one acquired signal resulting from theemission of electrical energy by the electrode, the acquired signalvarying with electrical contact between the electrode and the myocardiumto differentiate between electrical contact and the absence ofelectrical contact, a processing element electrically coupled to thesignal acquisition element to compare the acquired signal for theelectrode with an expected signal, and an output element for generatinga contact-indicating output for the electrode based upon the comparison,the contact-indicating output indicating the presence or absence ofelectrical contact between the electrode and the myocardium.
 19. Asystem according to claim 18wherein the acquired signal is anelectrogram.
 20. A system for evaluating electrical contact between themyocardium and multiple electrodes inside the heart comprisingan energygenerating element for electrical connection to the electrodes to emitelectrical energy into the myocardium in a predetermined sequencethrough at least two electrodes, a signal acquisition element adapted tobe electrically coupled to the electrodes for detecting, independentlyfor each emitting electrode, at least one acquired signal resulting fromthe emission of electrical energy by the respective electrode, theacquired signal varying with electrical contact between the respectiveelectrode and the myocardium to differentiate between the presence ofthe electrical contact and the absence of the electrical contact, aprocessing element electrically coupled to the signal acquisitionelement that compares, independently for each emitting electrode, theacquired signals for the respective electrode with an expected signal,and an output element for generating, independently for each emittingelectrode, a contact-indicating output for the respective electrodebased upon the comparison.
 21. A system according to claim 20wherein theenergy generating element generates electrical energy that activates themyocardium.
 22. A system according to claim 21wherein the acquiredsignal is an electrogram.
 23. A system according to claim 20wherein theenergy generating element generates electrical energy that does notactivate the myocardium.
 24. A system according to claim 23wherein theacquired signal is a tissue impedance measurement.
 25. A method forevaluating electrical contact between the myocardium and a multipleelectrode array inside the heart comprising the steps ofdeploying amultiple electrode array into a chamber of the heart, the arraycomprising first and second circumferentially spaced splines forcontacting circumferentially spaced endocardial regions, each splinesupporting at least one electrode, electrically sensing electricalcontact between at least one electrode on the first spline and themyocardium in a first endocardial region to generate a first unitarycontact-indicating output indicating the presence or absence of theelectrical contact in the first region, and electrically sensingelectrical contact between at least one electrode on the second splineand the myocardium in a second endocardial region circumferentiallyspaced from the first region to generate a second unitarycontact-indicating output indicating the presence or absence of theelectrical contact in the second region, wherein, in electricallysensing electrical contact, the respective electrode is caused to emitan electrical signal that activates the myocardium.
 26. A methodaccording to claim 25and further including the step of correlating thefirst and second unitary contact-indicating outputs to generate acompound contact-indicating output indicating the aggregate of theelectrical contact between the myocardium and the multiple electrodearray.
 27. A method according to claim 25wherein, in electricallysensing electrical contact, electrograms are sensed.
 28. A methodaccording to claim 25wherein the steps of electrically sensingelectrical contact comprises the steps ofemitting electrical energy intothe myocardium independently through the electrodes, and detecting,independently for each emitting electrode, at least one acquired signalresulting from the emission of electrical energy by the respectiveelectrode, the acquired signal varying with electrical contact betweenthe respective emitting electrode and the myocardium to differentiatebetween the presence of the electrical contact and the absence of theelectrical contact.
 29. A method according to claim 28wherein the stepsof generating the first and second unitary contact-indicating outputsinclude comparing the acquired signal for each emitting electrode withan expected signal to determine the presence or absence of theelectrical contact.
 30. A method according to claim 29wherein the stepsof generating the first and second unitary contact-indicating outputsinclude generating, independently for each emitting electrode, a unitarycontact-confirm output for the respective electrode if the acquiredsignal for the respective electrode compares with the expected signaland a unitary contact-contrary output for the respective electrode ifthe acquired signal does not compare with the expected signal.
 31. Amethod according to claim 25and further including the step ofconditioning at least one electrode on the first and second supportsplines to emit a therapeutic signal into the myocardium.
 32. A methodaccording to claim 25and further including the step of conditioning atleast one electrode on the first and second support splines to obtain adiagnostic signal from the myocardium.